The present invention relates generally to timing devices, and more specifically to an appliance timer having an apparatus for permitting quiet manual setting of the appliance timer.
Appliance timers are commonly used in many household appliances, such as dishwashers, clothes washers, and clothes dryers. The appliance timer controls operations of the appliance by actuating and deactuating switches which start and stop various work operations within the appliance such as a rinse operation in the case of a dishwasher. The switches within the appliance timer are actuated and deactuated by interaction of a cam surface defined in a cam of the appliance timer and a cam follower which is associated with a particular switch.
One common appliance timer is an interval drive timer. This type of appliance timer typically includes a number of vertically mounted cylindrical cams driven by a ratchet and drive pawl assembly. Each of the cams includes a cam profile defined in an outer surface thereof which selectively actuates one or more switches thereby controlling various work operations of the appliance. In operation, the drive pawl indexes the ratchet at predetermined intervals. Accordingly, the ratchet, and thus the cams attached thereto, are in motion for a first period of time. Thereafter, the ratchet is at rest for a second period of time until the next movement thereof by the drive pawl. For example, a one-minute interval timer may cause the ratchet to be in motion for five seconds and then at rest for 55 seconds.
It is desirable to control an entire cycle of the appliance with one 360.degree. rotation of the cam of the appliance timer. This feature enables the appliance timer to possess a reduced number of parts since only one cam would be necessary in the timer. In addition, this feature enables manual setting of the appliance timer to be simplified. This is true since any operational segment of the appliance cycle may be accessed by manually rotating a setting knob, which is operatively coupled to the cam, a rotational distance of less than 360.degree..
However, by controlling the entire cycle of the appliance with one 360.degree. rotation of the cam, the amount of cam surface which is available for actuating and deactuating switches within the appliance timer is limited. This becomes a problem since it is desirable to quickly rotate the cam so as to increase the accuracy of the timer. More specifically, the timer's ability to control timing accuracy of the duration of a work operation between the point in time at which the work operation begins and the point in time at which the work operation ends is directly proportional to rotational speed of the cam. This is true since dimensional errors (e.g. manufacturing errors) and design tolerances associated with the components of the timer (e.g. the cam profiles and the cam followers) remain constant regardless of rotational speed of the cam. For example, if the location of a drop along the cam profile associated with actuation of a particular work operation is placed 2.degree. further down the cam profile by a manufacturing error, the cam follower will be required to travel the additional 2.degree. prior to dropping, therefore delaying the actuation of the work operation. If the cam is rotating at a speed of 0.5.degree. per second, actuation of the work operation will be delayed by four seconds. However, if the cam is rotating at a speed of 4.degree. per second, actuation of the work operation will be delayed by only one-half second thereby improving the accuracy of the timer.
Thus, in the operation of an appliance timer, it should be appreciated that there exists a tension between the desire to quickly rotate a cam of an appliance timer so as to improve the accuracy of the timer, and the desire to slowly rotate the cam of the appliance timer so that the entire cycle of the appliance is controlled by one 360.degree. rotation of the cam.
Moreover, it is also desirable to provide an appliance timer which may be manually set by the operator in a quiet manner. More specifically, it is desirable to provide an appliance timer in which noise generating components (e.g. a number of cam followers and their associated cam profiles) are disengaged from one another during a period of time in which the operator manually sets the appliance timer. This disengagement of noise generating components reduces the noise associated with the appliance timer.
What is needed therefore is an appliance timer which rotates a cam at a high speed when actuating or deactuating switches within the appliance timer so as to more accurately control critical work operations, yet rotates the cam at a low speed when the appliance timer is actuating or deactuating switches of work operations which are not critical so that space on the surface of the cam is conserved. What is further needed is an appliance timer which has an apparatus for permitting quiet manual setting of the appliance timer.